The components of high-temperature mechanical systems, such as, for example, gas-turbine engines, must operate in severe environments. For example, the high-pressure turbine blades, vanes, blade tracks and blade shrouds exposed to hot gases in commercial aeronautical engines typically experience metal surface temperatures of about 1000° C., with short-term peaks as high as 1100° C.
Components of high-temperature mechanical systems may include a Ni- or Co-based superalloy substrate. The substrate can be coated with a thermal barrier coating (TBC) to reduce surface temperatures. The thermal barrier coating may include a thermally insulative ceramic topcoat, and may be bonded to the substrate by an underlying metallic bond coat.
The TBC, usually applied either by air plasma spraying or electron beam physical vapor deposition, is most often a layer of yttria-stabilized zirconia (YSZ) with a thickness of about 100-500 μm. The properties of YSZ include low thermal conductivity, high oxygen permeability, and a relatively high coefficient of thermal expansion. The YSZ TBC is also typically made “strain tolerant” and the thermal conductivity further lowered by depositing a structure that contains numerous pores and/or pathways.
Economic and environmental concerns, i.e., the desire for improved efficiency and reduced emissions, continue to drive the development of advanced gas turbine engines with higher inlet temperatures. In some cases, this may lead to the replacement of the superalloy substrate with a silicon-based ceramic or ceramic matrix composite (CMC) substrate. Silicon-based ceramics or CMCs possess excellent high temperature mechanical, physical and chemical properties, and may allow gas turbine engines to operate at higher temperatures than gas turbine engines having superalloy components.
However, silicon-based ceramics and CMCs suffer from recession in combustion environments due to the volatilization of silica by water vapor. Thus, silicon-based ceramic and CMC substrates may be coated by a substantially non-porous environmental barrier coating (EBC), which protects the substrate from environmental degradation, such as water vapor attack or corrosion.